CHAP. XX.] 



EXTRA LEGS : POSITION P. 



499 



posterior by several points. This may be seen in the spurs at 

 the apex of the tibia, for the anterior spur (a) is long, but the 



a— 



1 



II 



in 



Fig. 165. Silis ruficollis, No. 764. I. The right anterior leg seen from ventral 

 surface, a, anterior, p, posterior. This figure was drawn with the microscope 

 and is reversed. II. Detail of apex of tibia of the anterior or normal tibia, 

 shewing a, the anterior, and p, the posterior spurs. III. Similar detail of apex of 

 the tibia of the extra limb, shewing^' and p", two structurally posterior spurs. 



posterior (p) is short (as is usual in the front leg of many beetles). 

 The hairs on the surface of the tibia are also directed asymmetri- 

 cally and the parting or division between them is not median, 

 but is nearer to the anterior border (see figure). 



But in the extra part there is no such differentiation, and both 

 surfaces are structurally posterior surfaces. The hairs part in the 

 middle, and both spurs (p', p") are formed as posterior spurs. This 

 extra structure is therefore made up of the two posterior borders 

 of a right and a left tibia compounded together in Position P. 

 (See diagram, Fig. 165, II.) 



This specimen was found by Dr Sharp amongst a number 

 of insects collected by myself in his company at Wicken Fen on 

 Sunday 1 , July 26, 1891. 



Such a case taken in connexion with others (e.g. No. 801) 

 makes it certain that many cases of supposed " single " extra 

 appendages are really examples of double extra parts. 



1 A day or two before, the manuscript of this part of the subject had been put 

 by with the remark that no good opportunity of thoroughly investigating a case 

 of " single " extra leg had occurred, but that it could scarcely be doubted that traces 

 of duplicity would be found in them. Considering the great rarity of extra ap- 

 pendages in Insects, and remembering that even of the whole number very few are of 

 the supposed " single " order, I have thought the occurrence of this capture a 

 coincidence of sufficient interest to be worthy of mention. Dr Sharp tells me that 

 amongst all the beetles that have gone through his hands only one case of extra 

 appendage (No. 755) was seen. 



32—2 



